The present invention relates to a pneumatic tire having a tread surface which is provided with a tread pattern constituted by sipes, auxiliary grooves and lug grooves, the sizes of which are determined at specific ratios to the depth of the main groove. More specifically, the invention is concerned with all season tires which are suited to use both on a snowy road and a paved road.
In conventional snow tires, the tread pattern is designed such that both the grooves and the sipes have greater depths than those in ordinary tires. The conventional snow tires, therefore, exhibit a small rigidity and, hence, their steerability and stability are poor particularly on paved roads. This causes some problems particularly when the road is partly covered with snow such that the pavement appears discontinuously. Under these circumstances, studies have been made to develop snow tires which exhibit superior steerability and stability on paved roads, in order to allow vehicles to run both on snowy roads and roads where the pavement is revealed.
These tires, however, have not been improved so as to satisfactorily overcome the unfavourable effects produced by the tire rigidity on the tire performance on snowy and paved roads.
More specifically, in these snow tires, the numbers of tread grooves and sipes are increased so as to improve the tire performance on snowy roads and, at the same time, the depths of the tread grooves and the sipes are partially reduced so as to improve the performance on paved roads. This type of tires exhibit a satisfactory performance both on snowy and paved roads when the tire is still new. However, as the tire is worn down, the rigidity of the tread is drastically increased to cause a rapid lowering of the tire performance on snowy roads. Although a progressive impairment of the tire performance to some extent is unavoidable, a drastic lowering of the tire performance causes a serious problem from the view point of safety.